Final answer:
The levels of Cdks and cyclins change during the cell cycle because cyclin concentrations fluctuate while Cdk levels remain stable. Cdks activate proteins by phosphorylating them, advancing the cell to the next phase. Cyclins regulate the cell cycle when bound to Cdks, forming complexes at specific points to regulate checkpoints.
Step-by-step explanation:
The levels of Cdks and cyclins change during the cell cycle because the concentrations of cyclin proteins fluctuate while the levels of Cdk proteins remain relatively stable. Cdks are enzymes that phosphorylate other proteins, activating them and advancing the cell to the next phase. Cyclins regulate the cell cycle when they are tightly bound to Cdks, forming Cdk/cyclin complexes at specific points in the cell cycle to regulate different checkpoints.